This application relates generally to gas turbine engines and, more particularly, to gas turbine engine bearing assemblies.
A gas turbine engine typically includes a multi-stage axial compressor, a combustor, and a turbine. Airflow entering the compressor is compressed and directed to the combustor where it is mixed with fuel and ignited, producing hot combustion gases used to drive the turbine. The turbine is coupled to the compressor with a rotor shaft supported by a plurality of assemblies. As a result of hot combustion gases, the turbine, and rotor shaft are subjected to thermal stresses. Additionally, the bearing assemblies supporting the rotor shaft are, subjected to thermal conduction from contact with the rotor shaft.
As turbine engines have evolved, higher stage loading turbo-machinery, including larger bearing assemblies and rotor shafts, have been included within the engines to provide increased pressure ratio cycles for the turbine engines. Higher pressure ratios increase cycle temperatures and air temperatures within the engine. Specifically, higher stage loading causes an operating speed of the turbines to increase, resulting in temperature increases in the rotor and bearing assemblies.
To minimize the effects of increased pressure ratio cycles, known bearing assemblies include isolation systems that attempt to isolate the bearing assemblies from the rotor shaft. Isolation systems often include shaft oil cooling systems, increased bearing compliance or trilobing, reduced inner race to shaft fits, and decreased bearing support stiffness"". Other isolation systems have included bearing assemblies fabricated from various materials. Often such isolation systems result in only minimal operating life extensions of the bearings for increased pressure ratio cycles. As a result, the bearing systems have not been effectively isolated from the effects of the rotor shaft.
In an exemplary embodiment, a gas turbine engine includes a bearing isolation system that effectively isolates bearing assemblies from rotor shafts. The bearing isolation system includes an isolation shaft and a pair of pilots. The gas turbine engine includes a compressor impeller and a turbine assembly. One of the pilots is disposed a distance upstream from the bearing assemblies between the isolation shaft and the rotor shaft, and one of the pilots is disposed a distance downstream from the bearing assemblies between the isolation shaft and the rotor shaft. The isolation shaft is connected to the rotor shaft with the pilots and is disposed between the bearing assemblies and the rotor shaft such that an air gap is created between the isolation shaft and the rotor shaft. The bearing assemblies are mounted to the isolation shaft. The pilots are positioned axially along the rotor shaft and during engine operation, permit the isolation shaft to move axially with respect to the rotor shaft. Additionally, each pilot includes a plurality of openings to permit cooling air to flow within the gap defined between the isolation shaft and the rotor shaft.
During operation, cooling air is channeled through the pilot openings and into the gap disposed between the isolation shaft and the rotor shaft. The cooling air reduces heat transfer from the rotor shaft to the bearing assemblies and reduces an operating temperature of the isolation shaft and rotor shaft. Because the isolation shaft is connected to the rotor shaft with only the pair of pilots, direct heat conduction between the bearing assemblies and the rotor shaft is reduced in comparison to known bearing assemblies including bearings coupled directly adjacent a rotor shaft. Furthermore, because the pilots are spaced a distance from either side of the bearing, thermal and mechanical growth of the compressor impeller and turbine assembly is attenuated along the isolation shaft prior to reaching the bearing assemblies. As a result, thermal and mechanical growth of the bearing assembly is reduced, thus extending the operating life of the bearing assembly. Additionally, the pilots create thermal resistance between the rotor shaft and the isolation shaft to attenuate heat conduction between the rotor shaft and bearing inner races. As a result, the bearing assemblies are effectively isolated from the rotor shaft and mechanical capability and durability of each bearing assembly is improved.